 A parting shot is a hostile gesture or remark made while departing, which the person hearing has no chance to respond to. And it's a phrase I've always found interesting. It dates to a military tactic from the 17th century. The Parthians at the time were an ancient race living in northeast Persia, and their army included mounted archers. These archers would ride away from the enemy at full gallop, giving the impression of a retreat. But as the enemy approached, the archers, who were superb equestrians,